The invention relates to an imageable metal substrate, in particular to a coated planer substrate which can be laser imaged to form a printing plate.
Conventional lithographic printing plates, such as those typically used by both newspaper and commercial printers, are usually made of grained, anodized aluminum substrate which has been coated with a light sensitive coating. The grained, anodized aluminum is generally post treated to enhance hydrophilicity of the substrate sheet prior to the application of the light sensitive coating. Solutions which are useful for post treatment include, for example, sodium silicate and polyvinylphosphonic acid.
Graining of aluminum is accomplished in a variety of ways, including rotary brush graining, chemical graining and electrochemical graining. It is possible to use more than one of these techniques in the production of lithographic substrate. A grained surface has better adhesion to light sensitive coatings and carries fountain solution in the background areas of the plate on the press more efficiently than the ungrained surface.
Anodizing is the process of electrolytically generating aluminum oxide on the surface of the aluminum sheet. Commonly used anodizing electrolytes include sulfuric acid and phosphoric acid. Since anodic aluminum oxide is harder and more abrasion resistant than aluminum, an anodized printing plate has a greater press life than a bare plate.
Computer-to-plate systems using infrared lasers are now available for imaging printing plates. By imaging directly on the plate, the use of photographic negatives is eliminated.
U.S. Pat. No. 4,731,317 to Fromson et. al. discloses a printing plate based on a substrate which is brush grained in a slurry comprising alumina, followed by successive treatments in dilute sodium hydroxide and nitric acid, and subsequent anodizing to achieve an oxide coating weight of 1.5 milligrams per square inch. The substrate may also be silicated after anodizing to improve hydrophilicity in accordance with U.S. Pat. No. 3,181,461. The anodized plate is coated with a diazo resin which is transparent to the radiation of a YAG infrared laser (1064 nanometers), but is sensitive to the longer wavelengths generated within the areas of the anodic oxide exposed to the laser. The theory is that the grained surface traps the laser radiation and re-emits the energy as longer wavelengths. This light trapping property must be enhanced by the addition of carbon black to the diazo. The diazo is rendered insoluble where the plate is exposed to the laser. Following laser exposure, the unexposed diazo is removed with a solvent to reveal hydrophilic oxide in the background. Because the non-imaged areas are removed with a solvent, the plate is described as negative working.
U.S. Pat. No. 4,731,317 mentions that the diazo may partially ablate when the level of the laser radiation is relatively high. Such ablation is undesirable since the areas exposed to the laser radiation are to remain on the plate as the ink bearing image after processing in the developing solution.
According to the present invention, a planar or curved metal substrate is treated such that the surface is capable of being visibly imaged by selective writing with an infrared laser. A preferred treatment for this purpose is rotary brush graining. The phrase xe2x80x9crotary brush grainingxe2x80x9d is intended to refer to any process using axially rotating brushes that tangentially contact a surface to be grained in the presence of a slurry containing particulate material such as alumina, silica and the like. The phrase also includes equivalent processes that produce the same result.
The treated surface is coated with an ablatable coating which is transparent to imaging infrared laser radiation. Selective exposure to infrared laser radiation ablates this coating in the laser exposed areas as a result of the absorption of infrared radiation by the treated metal surface. The coated substrate can be imaged in a computer-to-plate infrared laser imaging device. Depending on the specific coating and substrate selection, the imaged substrate can be used in a conventional lithographic printing process or in a dryographic printing process.
The printing plate of the invention thus comprises a metal substrate with a laser ablatable coating thereon wherein the substrate itself can be imaged with a laser.
The preferred metal substrate is aluminum which is preferably anodized after being treated to render the substrate imageable by an infrared laser. Anodized aluminum may optionally be post treated with sodium silicate, polyvinylphosphonic acid or the like to enhance the hydrophilic nature of the non-image areas.
The ablatable coating itself does not absorb ablative infrared laser radiation, since it is transparent to it. The imaging infrared laser radiation passes through the coating and is absorbed by the treated metal substrate. The coating in the laser imaged areas ablates as a result of the incident infrared energy captured by the treated metal substrate. The coating in the areas not exposed to the imaging laser radiation remains adhered to the plate.
The substrate of the invention serves three functions. First, it carries an ablatable coating. Secondly it is capable of absorbing infrared laser radiation to ablate the coating. Lastly, it becomes the printing plate wherein the laser ablated areas function as the image or the background depending on the choice of coating and the mode of printing. i.e. lithographic or dryographic. Because the substrate itself causes laser ablation of the coating, which functions as the image or background after laser imaging, no intermediate layer or coating is required to promote or cause ablation to take place.
In a further embodiment, the ablatable coating is positive acting with respect to imaging by ultraviolet radiation. The infrared laser ablated (imaged) plate is blanket exposed to ultraviolet light to an extent sufficient to solubilize the ablation debris left behind in the background area without substantially affecting the image on the plate.